Why.
Also just in general, you are not going to see Eriana's Vow, glaives, or special sidearms in that mode at all with poor special ammo deliveries like that. You aren't one-shotting an enemy with a glaive, but you get just as much ammo as you would with a one-shot weapon. That is just poor ammo distribution.
It's no wonder it's so meta in Checkmate.
EDIT: The title of this post was updated to mention that it was Checkmate Trials specifically now that that particular Trials weekend has passed. This is just to clarify and prevent future confusion when reading this post.
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Didn't know glaives got less ammo. You'd think they'd get at least 1.5 times what fusions get. But they do have shields and melees so maybe that's why.